Gasoline used in engine combustion contains a large number of compounds, typically including dozens or hundreds of hydrocarbons plus alcohol. Each of these compounds may have different energy densities and knock properties or “octane” inherent to their chemical composition and structure.
The octane of a fuel is the ease at which the fuel auto-ignites. A fuel's octane may be classified by its tendency to ignite under variable pressure or temperature conditions. An octane rating is a standard procedure for quantifying the conditions at which a fuel auto-ignites without external ignition. Compounds with higher octane ratings may withstand greater temperature within a combustion chamber without auto-igniting. High torque demands may be met by increased airflow into a combustion chamber, thus a combustion chamber may have high pressure and temperature during high torque operation. If chamber conditions reach the auto-ignition conditions of the air-fuel mixture located therein, pre-ignition or engine knock may occur.
Hydrocarbon compounds having high octane ratings often have low energy density. In general, the combustion of an amount of high octane fuel will produce less energy than the combustion of the same amount of low octane fuel. Thus for a given energy demand, more high octane fuel is injected into a combustion chamber than low octane fuel. Thus the protective benefits of the high octane components of fuel are balanced with the fuel efficiency losses from combustion of fuels that are not energy dense.
Gasoline separation has been suggested as a means to address the above issues. Prior approaches have removed ethanol from blended fuel mixtures for selective injection. Ethanol however, is just one of the many high octane components of gasoline. Further, this method does not allow for a variable octane separation threshold.
Further approaches have separated the gasoline of an externally filled fuel tank into a low octane portion and a high octane portion stored separately in a high octane fuel tank and a low octane fuel tank. Many vehicles however have limited available space and therefore cannot accommodate a three fuel tank and three fuel pump configuration. Further, this method adds additional weight to the vehicle contributing to fuel efficiency losses, and the method is also high cost.
The inventors herein found that by separating fuel by octane level, the desired octane separation threshold may be set at a number of values to achieve precise combustion control. Further, by separating fuel into a high octane portion and a low octane portion and returning the low octane portion to the externally filled fuel tank (or vice versa) fuel separation advantages may be achieved without adding a third fuel pump and tank, lessening both the weight and space occupied by the separation system, and decreasing the system cost. A fuel separator may have a higher low octane output or a higher high octane output. Thus the separation tank may be smaller than the externally filled fuel tank and may store the fuel with an octane rate corresponding to lower separator output to further minimize the volume occupied by the separation system and the system weight and to effectively transform the externally filled fuel tank into a high octane fuel tank or a low octane fuel tank.
An exemplary embodiment may deliver fuel from an externally filled fuel tank to a separator where it may be separated into a low octane portion and a high octane portion. The high octane portion may be delivered to the high octane storage tank and the low octane portion may be returned to the externally filled fuel tank. The octane level within the externally filled fuel tank may continuously decrease throughout fuel separation and thus the octane level may be continuously monitored. An operating method may terminate fuel separation if the externally filled fuel tank's fuel level falls below a threshold or empties or if the smaller high octane fuel tank is full. Further embodiments may terminate separation if the octane level in the externally filled fuel tank falls below a threshold.
Prior fuel separation approaches experience fuel staleness after extended operation at a limited range of speed-loads. For example, this may result from a vehicle being used for heavy towing or operated at high power much more frequently than at lower power, thus low octane fuel may be used less frequently than high octane fuel. Alternatively, a vehicle may be operated almost exclusively at idle and light loads, thus high octane fuel may almost never be used. Therefore fuel in the underused tank may become stale after a period of time. Disclosed embodiments decrease or eliminate fuel staleness by independently monitoring conditions contributing to fuel staleness within the separate tanks. If fuel is determined to be stale, fuel from the underused tank may be delivered to the engine for combustion. Determination of fuel staleness may be desired for any engine system which stores and uses two fuels independently. For example, it may be desired for a dual fuel gasoline+CNG engine, for a gasoline PFI+E85 DI engine, for a system which uses onboard separation of ethanol from a gasoline-ethanol blend, etc.
The disclosed system is particularly well suited for systems equipped with secondary air injection for fast catalyst light-off and emission reduction. Exhaust gas enriched with an amount of low octane fuel may be more readily combustible than exhaust gas enriched with high octane fuel. Thus, when secondary air injection is desired low octane fuel may be used for increased secondary combustion efficiency.
In an exemplary embodiment, a system may have a high octane fuel tank and a low octane fuel tank and may be equipped with secondary air injection for exhaust combustion. When secondary air injection is desired an amount of low octane fuel delivered to the engine may increase. Similarly, if a high amount of low octane fuel is being used for combustion, secondary air injection may be initiated. Similar operation may also be used to increase efficiency and reduce emissions during engine cold start.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.